Guitars often have truss rods within their necks. A truss rod is an adjustable metal rod that goes down the inside center of the guitar neck. In many guitars the truss rod can be adjusted using a nut located at one end of the truss rod. This allows adjustment of the shape of the neck. This adjustment can be very handy, for example, when temperature or humidity changes, etc., can tend to warp or otherwise change the shape of the neck. An adjustment of the trust rod can allow the neck to be maintained at an optimal shape for playing the guitar.